1. Field of the Invention
The present invention relates to a compositionally graded sintered alloy and a method of producing the same, which sintered alloy is very suitable for cemented carbide, cermet, and substrate of coated sintered alloy produced by coating cemented carbide or cermet with a hard film, used for tools of various types represented by a cutting tool such as insert chip, drill and end mill, a wear-resistant tool such as die, punch and slitter, and a construction tool such as cutter bit.
2. Description of the Related Art
A hard sintered alloy such as a cemented carbide represented by WC—Co based alloy and WC—(W, Ti, Ta) C—Co based alloy and a cermet represented by TiC—Fe and Ti(C,N)—WC—TaC—Ni obtains alloy characteristics required in each of the applications of cutting tool or member, wear resistant tool or member and the like, which alloy characteristics includes hardness, strength, toughness, wear resistance, breaking resistance and chipping resistance, by adjusting the grain size of a carbide hard phase, the amount of a metal binder phase, and the amount to be added of other carbides (VC, Cr3C2, Mo2C, ZrC and the like) and thereby achieving performance required in use. However, hardness and toughness (or wear resistance and breaking resistance) are alloy characteristics which are incompatible with each other and it is very difficult to improve such two incompatible characteristics at the same time. As one example of a method of improving hardness and toughness at the same time, there has been proposed a method of reinforcing a binder phase of an alloy by adding Cr, Mo, Al, Si, Mn and the like (e.g., Japanese Patent Application Laid-Open (JP-A) No. 7-138691, Japanese PCT Patent Application Laid-Open 2000-503344 and JP-A 2001-81526). Further, there has also been proposed a method of changing the amount of binder phase, the amount of added carbide and the grain size of WC between a surface of a sintered alloy and an inner portion of the sintered alloy, thereby producing a compositionally graded structure in which a vicinity of an alloy surface has relatively high hardness and high wear resistance (or relatively high strength and high toughness) (e.g., JP-A 2-209448, JP-A 2-209499, JP-A 2-15139, JP-A 2-93036, JP-A 4-128330 and JP-A 4-187739). The method of producing a compositionally graded alloy aims at strengthening a region in the vicinity of an alloy surface which is to function as a tip of a tool blade, and thus is a reasonable and effective method.
As a method of improving the aforementioned alloy characteristics by adding a substance, JP-A 7-138691 discloses a cemented carbide for processing aluminum, comprising 5 to 35% by weight of Cr with respect to the amount of a metal binder phase, wherein the amount of the binder phase is 4 to 25% by weight of the weight of the alloy and the remainder is WC whose average particle diameter is 1 to 10 μm. Further, Japanese PCT Patent Application Laid-Open 2000-503344 discloses a cemented carbide as a minute composite material containing 3 to 30% by weight of a binder metal obtained by a sintering reaction effected in a micro wave region, which material further contains 0.01 to 5% by weight of Mo, Mn, Al, Si and Cu and in which the metal binder phase is comprised of Ni and Cr. Yet further, JP-A 2001-81526 discloses an iron-based cemented carbide comprising Fe whose binder phase comprises 0.35 to 3.0% by weight of C; 3.0 to 30.0% by weight of Mn and 3.0 to 25.0% by weight of Cr.
In each of the cemented carbides disclosed in the above-described three references, a good effect of improving the alloy characteristics as described above is not likely to be obtained because the content of a binder phase thereof is relatively low, although the binder phase thereof is strengthened due to addition of metals. Further, as these cemented carbides are basically alloys each having an even or non-graded composition (between a surface and an inner portion of an alloy), there still arises a problem in that the cemented carbides cannot improve hardness and toughness at the same time in a satisfactory manner.
On the other hand, as a method of improving the above-described alloy characteristics by a graded composition, JP-A 2-209448 and JP-A 2-209499 each disclose a cemented carbide having a surface region formed such that the content of a binder phase therein is lower than the content of a binder phase in an inner portion of the alloy or the cemented carbide. The cemented carbides disclosed in these two references achieve relatively high hardness due to a decrease in the content of a binder phase in the surface region but suffers from a decrease in toughness. Therefore, the cemented carbides still cannot improve hardness and toughness at the same time in a satisfactory manner. Further, in these cemented carbides, there arises another problem in that it is difficult to significantly reduce the content of a binder phase in a surface region thereof to make the content distribution of the binder phase graded.
Further, JP-A 2-15139 and JP-A 2-93036 each disclose a TiCN-based cermet in which a hard phase in the vicinity of a surface thereof is subjected to nitriding caused by sintering at a N2 partial pressure which is being adjusted, so that the content of a binder phase is decreased and the surface portion has higher toughness and hardness than an inner portion of the cermet. However, in the TiCN-based cermets of the above-described two references, although the wear resistance at the surface portions thereof are improved, the breaking-resistance thereof are not improved in a satisfactory manner. Thus, there arises a problem in that industrial fields to which these TiCN-based cermets are applicable are significantly restricted.
JP-A 4-128330 and JP-A 4-187739 disclose a cemented carbide and a cermet, respectively, in each of which, in a surface layer ranging from a surface to 0.2 to 10 mm inner side thereof, the content of at least one type of diffusion element selected from the group consisting of Cr, Mo, V, Ta, Al, Zr, Nb, Hf, W, Si, B, P and C gradually decreases from the surface toward an inner portion and the content of a binder phase or the particle diameter of a hard phase gradually increases from the surface toward the inner portion. In each of the cemented carbide and the cermet disclosed in the above-described two references, a region in the vicinity of a surface thereof has high hardness and good wear resistance due to a decrease in the content of a binder phase or the particle size of a hard phase being made fine, and also has high toughness, good breaking resistance and good plastic deformation resistance due to an effect in which a binder phase is made tougher, which effect is resulted from the presence of the above-described diffusion elements. However, the aforementioned cemented carbide and the cermet has a problem in that industrial fields to which these cemented carbide and the cerment are applicable are significantly restricted depending on the type of the diffusion element present at the surface layer.